1. Field of the Invention
The present invention relates to a water-in-oil (W/O) emulsion containing stabilized ascorbic acid, which is useful in the cosmetic, dermatological and/or veterinary fields, and methods for using these emulsions in the cosmetic treatment of the skin as well as for the preparation of a cream or ointment intended for the dermatological treatment of the skin and/or for veterinary treatment.
2. Discussion of the Background
Workers have long sought to stabilize ascorbic acid, or vitamin C, in suitable pharmaceutical forms, on account of its beneficial properties.
Indeed, ascorbic acid has many known biological functions, such as the stimulation of collagen synthesis, the strengthening of skin tissues against external attack (UV radiation, pollution), depigmentation, activity against free radicals and the compensation for vitamin E deficiency. Some of these beneficial properties have been reported in particular by England and Seifter in the article "The biochemical functions of ascorbic acid" published in Ann. Rev. Nutri., 1986: 6, pp 365-406.
However, owing to its chemical structure (alpha-keto lactone), ascorbic acid is very sensitive to the influence of environmental parameters such as light, oxygen and water (due to its pH and due to the presence of traces of metals). An unavoidable degradation of ascorbic acid in solution occurs over time.
This problem has been addressed in a variety of ways in the art.
In order to reduce or delay the degradation of ascorbic acid in solution, US-A-5,140,043 recommends stabilization by introducing ascorbic acid into aqueous-alcoholic solutions, formed of at least 80% water and having a pH below 3.5.
On account of the high acidity of these solutions, their use in the cosmetic and/or pharmaceutical field is not readily conceivable. Indeed, repeated application of these solutions may disrupt the equilibrium of the skin and may in particular irritate, or even burn, the skin.
Moreover, B. R. Hajratwala, in "Stability of ascorbic acid", published in the Revue Sciences Pharmaceutiques on 15 Mar., 1905, teaches that ascorbic acid may be stabilized as an acidic aqueous solution by adding a surface-active agent which is an oxyethylenated sorbitan ester.
In particular, Hajratwala states that at pH=3.4 and 25.degree. C., the addition of this agent reduced the rate of oxidation, and thus the rate of degradation, of ascorbic acid in solution.
Hajratwala also teaches the use of a chelating agent such as ethylenediaminetetraacetic acid (EDTA) and packaging under nitrogen, in the absence of light, in order to enhance the stability of the aqueous ascorbic acid solution.
However, such an acidic aqueous solution, applied to the skin, has the same drawbacks as those described above for acidic aqueous-alcoholic solutions. Furthermore, the stabilization obtained is still insufficient.
Other ways of stabilizing ascorbic acid have been proposed, in particular by a coating technique (FR-A-1,600,826) or by granulation of ascorbic acid (JP-A-53-127,819) for the agri-foods industry.
However, these techniques are, on the one hand, expensive and may, on the other hand, damage the ascorbic acid, for example during heating, and/or may lead to compositions of poor cosmetic acceptability, as in the case of granules.
Moreover, FR-A-1,489,249 discloses the use of metal salts of phosphorylated ascorbic acid, in particular magnesium ascorbylphosphate, in cosmetic compositions.
The latter compound has an activity close to that of ascorbic acid, from which it is derived, but it has certain drawbacks which render its use on the skin unfeasible. In particular, since magnesium ascorbylphosphate is only stable at basic pH (pH 8 to pH 9), it must be incorporated into a basic composition which may be irritant to the skin (the pH of which is about 5.5).
Consequently, none of the previous proposals have made it possible to overcome the technical problem associated with the instability of ascorbic acid in solution, in a pharmaceutical form which is suitable for the cosmetic and/or dermatological fields and at a cost which is compatible with industrial requirements.